They made the mistake of going too slow, hence their car kept kicking into 5th gear.

That's interesting - I had the same experience yesterday and learned about the "going too slowly" mistake. That was unexpected!

Because I was emphasizing not rushing in to situations where she would be forced to brake, she started over-thinking things - sometimes driving a bit too slowly, or beginning to coast too soon, and a couple of times it cost her extra fuel.

EG: We were slowly approaching a red light where we were going to turn left. There is a brief (2-3 car) advanced left-turn signal on these lights. I started to say, "it is possible to over-do things and go too slowly..." when the left turn arrow came on while we were still half a block away. We missed the left turn signal and had to wait through an entire light cycle because there was a steady stream of oncoming cars. Oops!

Of the 3 laps we did on this route -- 2 hers, 1 mine -- my lap happened to have the highest average speed, and the lowest fuel consumption.